You need to use vector tools on vectors. The gradient tool is not a vector tool. It can be used on the layer content that has the vector mask and on that layer raster layer mask but not on the layers vector mask. Here I added a spectrum gradient to the layer content a black to white gradient to the layer mask and am working with the direct selection tool on the vector layer mask path.

I think I may have not explained my situation well enough. Hopefully this helps clarify, which will help you to answer again if you can. Thanks for replying! You seem to know a lot and I am really hopeful that you can help me out.

I'm working on a photo, which is the background layer. I've created a basic action that lightens my image. This of course is a new layer and I don't want the whole image lightened, just parts. Once I click on that action and the new layer is created, I then hit the vector mask button in my lower layer tab, to create a mask, which I then use the gradient tool to adjust accordingly. In my example, I use this to then only lighten a part of the photo either with the side to side gradient or the circle gradient. I apologize for the lack of proper terms! This has always worked in the past.

I think the problem has something to do with the gradient not working. Even if I put a solid shape in a new layer, it does nothing when I try to use the gradient on that.

I believe this is what you have in mind. It consists of the Background layer image, a lightened layer above it, and a conventional (not vector) mask that when painted locally in solid or lesser value of white, reveals the lighter value.

I neglected to mention that if you want a gradient in the chosen mask area, paint the mask in white. Then Opt+click on the mask in the Layers panel to reveal the mask. Select the white area and use the gradient tool -- as illustrated here:

I'm not at home to screen shot, but the above image is what I put over the layer I want a gradient on. I've never used the mask and had to paint. I think instead of paint, I've just always chosen gradient by the paint fill option.

No help. His instruction to click on the Mask symbol will create a conventional mask. Should you want a Vector mask you must hold down the Cmd key as you click on that mask symbol. The confusion, I believe, is that he is erroneously calling a conventional mask a Vector mask. In addition his red callout note covered a critical part of the Layers panel and as such is of little value.

Ok. For the vector mask, I am/have been using the gradient tool, so any idea why that isn't working? Even when I just add a shape and a gradient to that it isn't working, so wondering if it's the gradient tool and not so much the vector mask?

You have most likely been using a normal raster mask not a vector mask. For the gradient tool to work on the mask the mask need to be Photoshop's target. You need to be aware of what is targeted in the layers Palette. Here a simple Layers Palette one layer with layer mask and and vector mask. The masks are unlinked from the layer's content note there are no chain icons just three icons. The Layer Content Icon, The Layer Mask Icon and the Vector Mask Icon. Note the different state if these icons in the following screen captures. You would be wise to include screen captures with your questions in the future. You can not use the gradient tool on a vector however you can fill a vector shape with a gradient fill. So the shape layer content would be filled with a gradient its vector mask will only be a vector shape, A Path.